1. Field of the Invention
The present invention relates to a method of manufacturing a rare-earth magnet comprising a magnet body including a rare-earth element, and a first protective film including nickel and a second protective film including nickel and sulfur, which are laminated in this order on the magnet body, and a plating bath used for the method.
2. Description of the Related Art
As a rare-earth magnet, for example, a Sm—Co5 system, a Sm2—Co17 system, a Sm—Fe—N system, or a R—Fe—B system (R represents a rare-earth element) is known, and is used as a high-performance permanent magnet. Among them, attention is specifically given to the R—Fe—B system, because the R—Fe—B system uses neodymium (Nd) which is more abundant and relatively less expensive than samarium (Sm) as a main rare-earth element, and iron (Fe) which is also less expensive, and the R—Fe—B system has magnetic performance equal to or higher than the Sm—Co systems and the like.
However, the R—Fe—B system rare-earth magnet includes an easily oxidized rare-earth element as a main component and iron, so the corrosion resistance thereof is relatively low, thereby problems such as degradation and variations in performance arise.
In order to improve the corrosion resistance of the rare-earth magnet, a magnet on which various corrosion-resistant protective films are formed has been proposed (refer to Japanese Unexamined Patent Application Publication No. Sho 60-54406 or Hei 9-7810).
Although the corrosion resistance of the rare-earth magnet can be surely improved by the protective films, further improvement has been required. For example, there is a problem that the result of a salt spray test on a protective film made of metal or an alloy disclosed in Japanese Unexamined Patent Application Publication No. Sho 60-54406 is not satisfactory, so it is difficult for the rare-earth magnet to obtain sufficient corrosion resistance.
Moreover, the R—Fe—B system rare-earth magnet mainly includes a main phase, a rare-earth-rich phase and a boron-rich phase, so in the case where a protective film is formed through plating, when the R—Fe—B system rare-earth magnet comes into contact with a plating bath, the rare-earth-rich phase with an extremely low oxidation-reduction potential forms a local cell with the main phase or the boron-rich phase. Further, in the case of a nickel-plating bath, immersion plating in which the rare-earth-rich phase with a low oxidation-reduction potential is leached out, and nickel with a high oxidation-reduction potential is electrodeposited occurs. The rare-earth-rich phase is present in a grain boundary of the main phase, so when the rare-earth-rich phase is leached out, grain boundary corrosion will occur in the R—Fe—B system rare-earth magnet. It is difficult to plate a corroded portion, and even if a nickel-plating layer is formed through electroplating, it is difficult to completely cover the corroded portion, because leaching of the rare-earth-rich phase is local corrosion. Industrially, the locally corroded portion is forcefully covered with a plating film with a thickness of 10 μm or over; however, when the portion is not sufficiently covered, pinholes are produced in a protective film, so a problem that sufficient corrosion resistance cannot be obtained arises.